1. Field of the Invention
The present invention relates to a communication system that comprises a plurality of nodes connected in common to transmission lines.
2. Description of the Related Art
As shown in FIG. 1, in a prior-art communication system, two-wire transmission lines 1, 2 are connected with transmission/reception circuits 31 through 3n as a plurality of nodes. All the transmission/reception circuits 31 through 3n comprise the same components. Positive potential Vcc (for example, 5V) is supplied to one end of the transmission line 1 via a terminal resistor 4 and positive potential Vcc is supplied to the other end via a terminal resistor 5 in the same way. Ground potential Vg (for example, 0V) is supplied to one end of the transmission line 2 via a terminal resistor 6 and ground potential Vg is supplied to the other end via a terminal resistor 7 in the same way.
In the transmission/reception circuit 31 a two-way I/O filter 11 is connected to the transmission lines 1, 2 via a connector 12. Connecting terminals A1, A2 are provided for connecting the I/O filter 11 to the transmission lines 1, 2 and connecting terminals B1, B2 arranged as opposed to the connecting terminals A1, A2. A transmission signal is individually supplied to the connecting terminals B1, B2 via a non-inverting amplifier circuit 13 and an inverting amplifier circuit 14. In addition, bias circuits 17, 18 are connected to the connecting terminals B1, B2 of the filter 11 via AC coupling circuits 15, 16 which comprise resistors 15a, 16a and capacitors 15b, 16b, respectively. Each of the signals provided by the bias circuits 17, 18 serves as a reception signal via a comparator 19 comprising a differential amplifier.
Upon outputting the transmission signal, the signal is amplified by the non-inverting amplifier circuit 13 and amplified in an inverted manner by the inverting amplifier circuit 14 as well. Transmission signals having opposite phases to each other are supplied to the filter 11 from the non-inverting amplifier circuit 13 and the inverting amplifier circuit 14. The filter 11 serves as a low-pass filter to allow the transmission signals to pass individually therethrough. An output transmission signal from the non-inverting amplifier circuit 13 passes through the filter 11 and is thereafter supplied to the transmission line 2. An output transmission signal from the inverting amplifier circuit 14 passes through the filter 11 and is thereafter supplied to the transmission line 1.
On the other hand, the information signals transmitted through each of the transmission lines 1, 2 are supplied to the filter 11. The filter 11 acts as a low-pass filter on each of these information signals to output signals to the AC coupling circuits 15, 16. Each of the AC coupling circuits 15, 16 extracts AC components of the information signals and supplies the components to the bias circuits 17, 18, respectively.
For example, as shown in FIG. 2A, consider the case where a signal A transmitted through the transmission line 1 and a signal B transmitted through the transmission line 2 vary in phase opposite to each other. As shown in FIG. 2B, the bias circuit 17 applies a bias voltage to the information signal A to obtain a biased signal BIASA, while the bias circuit 18 applies a bias voltage to the information signal B to obtain a biased signal BIASB. As shown in FIG. 2C, the comparator 19 detects each of the output signals BIASA, BIASB from the bias circuits 17, 18 as a reception signal.
When a break has occurred in the transmission line 1, only the signal B is transmitted through the transmission line 2. Accordingly, as shown in FIG. 2D, the biased signal BIASA remains constant, whereas the biased signal BIASB to the signal B, transmitted through the transmission line 2, to which a bias voltage has been applied changes in the same way as the signal B. The comparator 19 compares the constant biased signal BIASA with the biased signal BIASB to obtain a reception signal as shown in FIG. 2E. This holds true even when the transmission line 1 is grounded or when the transmission line 2 is broken or grounded.
Furthermore, no reception signals could be detected without the bias circuits 17, 18 when a break occurred in the transmission line 1 since the signals A, B to be inputted into the comparator 19 would have the waveforms shown in FIG. 2F.
Other transmission/reception circuits 32 through 3n also have the same configuration and operation as those of the transmission/reception circuit 31. Furthermore, the aforementioned prior-art communication system is disclosed, for example, in Japanese Patent Laid-Open Publications No.Hei 3-171849. In addition, such a system as has the aforementioned AC coupling circuits 15, 16 at the input stage of the receiver circuit is disclosed, for example, in Japanese Patent Laid-Open Publications No.Hei 1-317007 and No.Hei 1-261047.
As described above, the transmission/reception circuit is provided, at the input stage of the receiver circuit portion thereof, with the AC coupling circuits 15, 16 to extract from an information signal transmitted only desired frequency components that include information regarding each of the bits given when transmitted. However, the AC coupling circuit comprises a time constant circuit with a resistor and a capacitor connected in series, so that the time constant given by the resistor and the capacitor exert an effect on the input information signal. That is, a large time-constant would cause the passing frequency bandwidth to become broad but the response to the input information signal to become slow. On the other hand, a small time-constant would cause the passing frequency bandwidth to become narrow but the response to the input information signal to become quick. As shown in FIG. 3A and FIG. 4A, consider the case where the input information signal is a square wave of one bit, short in terms of time, and the time constant is large. In this case, the output signal waveform of the AC coupling circuit changes in a transient manner at the time of rising and falling as shown in FIG. 3B, so that a square wave cannot be obtained. On the other hand, if the time constant is small, the output signal waveform of the AC coupling circuit will be given a square wave with sharp rising and falling edges as shown in FIG. 4B.
As shown in FIG. 5A and FIG. 6A, consider the case where the input information signal is a square wave of a plurality of bits, long in terms of time and having a continuous high level, and the time constant is large. In this case, the output signal waveform of the AC coupling circuit changes in a transient manner at the time of rising and falling as shown in FIG. 5B, however, a square wave can be obtained since a constant level corresponding to the high level is sustained. On the other hand, if the time constant is small, the output signal waveform of the AC coupling circuit will be given a square wave with sharp rising and falling edges as shown in FIG. 6B. However, since the level of the waveform is gradually reduced from the rising edge to the falling edge, a square wave cannot be obtained.
The information signal consists of a train of bits and the passing frequency bandwidth needs to be broadened in consideration of the bits having a continuous logic xe2x80x9c1xe2x80x9d level corresponding to the high level in the train of bits. Generally, the time constant of the AC coupling circuit is given a large value in accordance with the maximum number of bits that have a continuous logic xe2x80x9c1xe2x80x9d level in the train of bits. However, as can be seen from the foregoing, the passing frequency bandwith becomes broad but the response to the input information signal become slow when the time constant is large. Accordingly, the square wave portion formed only of a bit with logic xe2x80x9c1xe2x80x9d level in the information signal changes in a transient manner at time of rising and falling and thus a square wave cannot be given. Therefore, when such an AC coupling circuit is provided at the input stage of the receiver circuit portion, no accurate reception signal can be obtained and thus high-speed communications cannot be achieved.
In view of the aforementioned circumstances, the object of the present invention is to provide a receiver circuit that can obtain a reception signal with accuracy in high-speed communications using two-wire transmission lines.
The receiver circuit according to the present invention receives information signals in a communication system with two-wire transmission lines for transmitting the information signals in opposite phase to each other. The receiver circuit comprises an alternating-current coupling means provided for each of the two-wire transmission lines and for extracting alternating-current components of the information signals inputted through the transmission lines. The receiver circuit also comprises a reception signal generating means for obtaining a reception signal corresponding to the information signals in accordance with output signals from each of the alternating-current coupling means. Each of the alternating-current coupling means is comprised of two time constant circuits having time constants different from each other and connected in parallel to each other in between an input and output thereof.
According to the receiver circuit of the present invention, each of the AC coupling means comprises two time-constant circuits, connected in parallel to each other in between the input and output thereof with time constants different from each other. Accordingly, each of the AC coupling means allows information signals with different frequency bandwidths to pass through each of the two time constant circuits and to be combined into an output signal to be outputted. Since the resulting combined signal has a waveform generally the same as that of the input signal that is inputted into the AC coupling means, the reception signal can be correctly obtained in high-speed communications using two-wire transmission lines. Furthermore, in the receiver circuit of a communication system of the present invention, the time constant of one of the two time constant circuits is made smaller according to the minimum number of bits having a continuous logic xe2x80x9c1xe2x80x9d level in a train of bits in an information signal transmitted. On the other hand, the time constant of the other one of the two time constant circuits is made larger according to the maximum number of bits having a continuous logic xe2x80x9c1xe2x80x9d level in a train of bits in a transmitted information signal.